1. FIELD OF THE INVENTION
This invention relates to magnetizing means for permanent magnet motors, and more particularly to a permanent magnet magnetizing device suitable for use in electric vehicles.
2. DESCRIPTION OF THE PRIOR ART
Permanent magnets of permanent magnet motors are generally magnetized by a stationary magnetizing equipment. However, inspections including checking of the degree of demagnetization of the permanent magnets are commonly rarely done when the permanent magnet motors are incorporated as a part of various machines. The permanent magnets of these permanent magnet motors are designed so as not to be easily demagnetized in an ordinary operating condition. However, in the case of the permanent magnet motors mounted on electric vehicles, great overload capacity is generally required to deal with overload encountered during, for example, acceleration and ascending an incline.
In the electric vehicle driven by the permanent magnet motor, direct application of non-controlled dc power supply voltage to the motor due to current commutation failure or breakdown of a thyristor chopper results in the flow of maximum armature current, and the armature reaction is increased to cause undesirable demagnetization of the permanent magnet in the field system. This undesirable demagnetization of the permanent magnet is also given rise to by natural deterioration with time. When the permanent magnet motor is operated in the state in which the permanent magnet in the field system is demagnetized for the reason above described, the required maximum torque cannot be developed due to the weakened field strength, and armature current excessively greater than that supplied in the non-demagnetized state of the permanent magnet must be supplied for developing the required motor torque, resulting in generation of excessive heat in the motor tending to impart damage to the permanent magnet. Thus, the acceleration characteristic and incline ascending characteristic of the electric vehicle will be extremely degraded to reduce the operating efficiency, with the result that the distance through which the electric vehicle can run without any make-up of the charge will become shorter than when the permanent magnet is free from demagnetization, and the operating performance of the electric vehicle will be extremely lowered to such an extent that the electric vehicle cannot properly operate.
A magnetizing equipment has been generally provided separately from the permanent magnet motor for the maintenance of the permanent magnet in the field system of the motor. For the magnetization of the permanent magnet subjected to demagnetization, the motor is disassembled and the magnetizing equipment is used for magnetizing the permanent magnet removed from the motor. Further, a slightly simplified method has been employed hitherto in which the armature winding of the permanent magnet motor is utilized for magnetizing the permanent magnet subjected to demagnetization. However, maintenance of the permanent magnet according to these prior art methods has been defective and very difficult in that the magnetizing equipment used for the magnetization of the permanent magnet is quite expensive and bulky, that the wiring of the electric vehicle must be disconnected in order to connect the motor to the magnetizing equipment, and that an ac power source is required for the operation of the magnetizing equipment.
A permanent magnet having a large coercive force and a high residual magnetic flux density is preferable in order that it can withstand demagnetization and the size and weight thereof can be reduced. However, such a permanent magnet is too expensive to be practically put into commercial use.